Plasma etch equipment

ABSTRACT

A microwave powered electron cyclotron resonance reactor employing a low pressure, high electron density plasma for rapid oxide etching using hydrogen and argon incorporates an alumina-coated quartz dielectric microwave window to couple microwave energy into an etch chamber while preventing oxygen in the window from contaminating the etch chamber or its contents. The etch chamber side of the dielectric microwave window is coated with alumina.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.07/818,662 filed Jan. 9, 1992, now U.S. Pat. No. 5,376,223.

BACKGROUND OF THE INVENTION

The present invention relates to the manufacture of semiconductorintegrated circuits, and more particularly to apparatus for ultra highpurity plasma cleaning and ultra pure material epitaxy.

Ion bombardment of surfaces is an important aspect of plasma etching,reactive ion etching, and sputter deposition. Ion bombardment also is animportant part of several analytical techniques such as secondary ionmass spectrometry (SIMS) and low energy ion scattering spectroscopy(LEIS). Auger electron spectrometry (AES) also employs low energy inertgas ions for physical sputtering to obtain erosion to support depthprofiling. Although these have become very important processes, the ioninduced surface chemistry and alteration of near surface chemical,physical and electronic characteristics are very complex and remainpoorly understood processes. This has led to the empirical developmentof most commercial plasma assisted etching processes.

Plasmas are partially ionized quasi-neutral gases. They can be createdin a vacuum chamber by applying enough electric field to ionize thegases. The power source may be a DC electric field, inductive RF coil,microwaves or a capacitively coupled RF electric field. Electrons havesmall mass relative to other particles so most of the energy gained inthese systems is initially absorbed by the electrons. These high energyelectrons collide with other particles and ionize the gas and sustainthe plasma. The typical ionizing potential is high so that the majorityof molecules and atoms stay neutral. Eventually a DC potential willbuild up between a plasma and any dielectric surface nearby, preventingany further imbalance.

In general, semiconductor processing plasmas are in a state of thermalnon-equilibrium and are affected by the following: (a) Power--Byincreasing power absorbed the sheath potential is increased as well asthe number of ions produced. Any wafer within the plasma will experienceinduced temperature increases from increased ion energy bombardment aswell as increased ion flux. Obviously, more damage can be done to thesubstrate at higher powers.

(b) Pressure--At higher pressures, more gas molecules are availablewhich is generally believed to result in higher ion flux. However, inour work we discovered a very sharp peak in production of excitedhydrogen species at low pressure in our electron cyclotron resonance,(ECR) plasma generator. This discovery has enabled us to take advantageof this peak in obtaining very fast etching at low ion energy so that weavoid surface damage usually occurring with energetic ions. While suchsurface damage is expected with heavy ions, it has also been documentedwith light ions such as H⁺.

(c) Device configuration--In ECR devices, parameters such as chambergeometry including positioning of the substrate, magnet configurationchamber materials and ion density uniformity will affect the etchingprocess.

It is postulated that the explanation for the distinctly differentetching rates of materials by various etchant gases has to do with theability of the reactive gas molecule to penetrate into the surface beingetched and to break the subsurface bond or lower the binding energy forthe surface atoms and to replace that bond by bonding itself to thesubsurface such that the released product is volatile at the temperatureof release. It is believed that when an energetic ion strikes a solid,it transfers its energy to near surface atoms through a series ofelastic collisions and electronic and vibrational processes. Collisionalcascade effects can produce ion implantation, crystalline damage, ionmixing and physical sputtering. These effects can also result from lowenergy ion bombardment. Ion mixing is the process under which targetatoms are relocated by ion impact, which process is broken up by recoiland cascade contributions. It is believed that mixing processes may beimportant in enhancing volatile product formation. This is distinct fromsputtering in which near surface atoms receive enough momentum transferperpendicular to the surface to overcome the surface potential barrierand thereby escape into the vacuum.

Two types of equipment are being most frequently used for plasmaetching. One type of such apparatus employs a resonant cavity excited byRF fields to induce and sustain the plasma. These are relatively simpledevices but a difficulty that they present is plasma density and ionenergy cannot be separately controlled. Accordingly high ion energiescannot be curtailed in high power requirement situations. Theconventional RF field, inductive or capacitively coupled, ischaracterized by low electron density and high plasma potential. Anothertype of plasma producing apparatus employs a microwave resonant cavityto produce plasma which is flowed out of the cavity to a remote area.This is called a downstream microwave plasma. Downstream microwaveplasma is characterized by low electron density, low plasma potential,and high plasma pressure. For this reason, electron cyclotron resonance(ECR) microwave plasma apparatus have become more popular for etchingapplication. The ECR apparatus can provide low pressure, high iondensity at low ion energy and the ion energy can be controlled bysubstrate bias and the plasma potential is also low.

An electron in motion in a magnetic field is acted upon by the field toproduce a force at right angles to the direction of motion of theelectron. As a result, an electron entering a fixed magnetic field willfollow a curved path. The radius of curvature is an inverse function ofthe intensity of the magnetic field. The frequency of electron rotation,w, is expressed as w=2.8×10⁶ B cycles/see where B is in gauss. This isknown as the electron cyclotron resonance frequency. We have designedour ECR plasma generator to employ a magnetic field of 875 gauss and thecorresponding cyclotron frequency of 2.45 GHz.

In recent years, the demands for reducing line widths and increasingdevice density in integrated circuits has forced the industry towards amanufacturing device called the integrated cluster tool. The integratedcluster tool is a multichamber vacuum system, in which the workingchambers are arranged around a central transfer chamber and in whicheach working chamber is separated from the central transfer chamber by apair of gate valves forming a vacuum lock. During operation, asemiconductor wafer can be processed in a working chamber, while theremainder of the cluster apparatus is isolated from the environment ofany of the other working chambers. After a wafer treatment is completedin a particular working chamber, the wafer is able to be automaticallypassed to the transfer chamber through a double gate valve and thenautomatically passed to a subsequent working chamber through anotherdouble gate valve. This integrated cluster tool permits a plurality ofvarious vacuum working chambers to be "clustered" around the centraltransfer chamber and permits the processing of a wafer through many ofits most demanding processes without any requirement for the wafer to bepassed back into ambient or clean room air. It has been proven that itis impractical and almost impossible, to control the particulate countin a clean room to the tolerances demanded by the density of modernintegrated circuits.

Because of the increasing importance of cluster tools, it is becomingcommercially important to decrease the time required for each processstep on a wafer. In the past, many wafers were processed simultaneouslyin large furnaces. Because of the high vacuum requirements andmechanical transfer requirements, a cluster tool working modulegenerally processes only one wafer at a time. Although duplicateidentical modules can be clustered around a transfer chamber, it is seenthat the cluster tool device is essentially a serial processing systemand that the process time of each step will have a direct affect on theoverall throughput rate.

SUMMARY OF THE INVENTION

We have discovered a process using a high density, low sheath potential,low pressure H₂ or H₂ /Ar plasma to very rapidly etch silicon oxide fromsilicon while passivating the silicon surface. Our process willcompletely remove native oxide from a silicon surface, without damage tothe subsurface, in less than 60 seconds. We have determined that it isimportant to remove from or passivate sources of oxygen in the plasmachamber. To this end we have coated the microwave window of our ECRchamber. By combining our ECR oxide cleaning processes with anintegrated cluster tool, we are able to pass an ultra clean Si wafer toa nitriding furnace without oxide regrowth and then subsequently grow astoichiometric Si₃ N₄ layer on Si which exceeds the purity of previouslygrown silicon nitride layers.

It is an object of this invention to provide an improved ECR process forlow temperature, rapid ECR plasma cleaning of silicon oxide.

It is a further objective to provide a silicon oxide cleaning processwhich is fast enough to support commercial production of integratedcircuit wafers in an integrated cluster tool.

It is a still further objective to provide an ECR silicon oxide cleaningprocess which in conjunction with a cluster tool and a nitride furnaceenables a process for the growth of stoichiometric Si₃ N₄ films withless than 0.01 at % oxygen, hydrogen and carbon.

DESCRIPTION OF DRAWING

FIG. 1 is a schematic of an ECR plasma apparatus.

FIG. 2 is a diagram of a typical integrated cluster tool.

FIG. 3 is a microwave window for the ECR apparatus.

FIG. 4 is a curve showing excited H density as a function of plasmapressure.

FIG. 5 is a curve showing O/Si ratio as a function of plasma pressure.

FIG. 6 is a curve showing O/Si ratio as a function of microwave power.

FIG. 7 is a curve showing O removal time as a function of Ar/H atomicpercent ratio.

DETAILED DESCRIPTION OF THE INVENTION

Those of ordinary skill in the art will realize that the followingdescription of the present invention is illustrative only and is notintended to be in any way limiting. Other embodiments of the inventionwill readily suggest themselves to such skilled persons from anexamination of the within disclosure.

With reference to FIG. 1, there is shown a schematic diagram of a plasmachamber device improved by us and used in our inventive processes. Weneed a low pressure, high density electron, low sheath potential plasmafor our work and we have employed an ECR reactor to provide the plasma.Electron cyclotron resonance is induced in chamber 1 by the impositionof microwave at the proper cyclotron resonance frequency from RFgenerator 3 via waveguide 5, past tuner 31 where the microwaves areintroduced into the chamber through microwave dielectric window 4. Themagnetic field from magnet 2 causes the electrons in the gases inchamber 1 to rotate at the same frequency as the microwaves such thatthe electrons are very quickly able to absorb energy from themicrowaves. One magnet coil 2 is shown for illustration purposes. Morethan one such magnetic is employed in our preferred embodiment but thisis not significant in respect to our invention. Excited electronsviolently impact molecules of any gas in the chamber and if conditionsof pressure and power are correct, many gas molecules will becomeionized, producing a quasi-neutral plasma wherein some molecules areionized by having their electrons knocked off and other molecules areslightly energized while remaining uncharged. The gases for sustainingthe plasma and for supporting the various reactions which take place inthe chamber 1 are introduced through one or more tube 13 into thechamber. Surrounding the chamber 1 is an annular fluid chamber 33 forcooling the chamber 1. Tubes 12 and 14 carry the coolant fluid in andout of the coolant chambers. The gas input tube 13 is shown connected toa mixing "T", 17, which is coupled two or more mass flow controllers 15and 16 connected in parallel for controlling the flow rate from gassources connected to two or more corresponding input lines 18 and 19.The wafer 10 is shown mounted to a substrate holder 9, which may havecoolant and heating conduits (not shown), both of which are mounted inthe plasma column. The axial position of the substrate holder 9 isadjustable and is an empirical adjustment for each gas employed. A fieldfocussing magnet 20 may be mounted beneath the substrate holder 9 tochange the magnet field lines in an effort to make the ion flux densityuniform across the chamber diameter in the treatment area. Shownconnected to the bottom of the chamber 1 is a large capacity high vacuumpump 8. Also shown connected to the substrate holder via electricalconnector 21 are the electrical circuits for biasing the wafer 10. An RFbias generator 23 and an RF matching network 22 are connected inparallel with a DC bias circuit containing an inductor 25 and a DCpotential source 26. Also shown is the interconnecting transfer chamberto the cluster tool of FIG. 2 which contains a pair of vacuum gatevalves 30 and 6 for separating the cluster tool from the ECR plasmachamber 1. All wafers 10 are introduced and removed from the chamber 1by a transfer arm from the cluster tool 41 (FIG. 2) which passes throughthe pair of gate valves, 30 and 6. With reference to FIG. 2, the ECR 42is shown connected to a modern cluster tool 41 for passing wafers backand forth for processing. Shown connected to the cluster tool through adual gate valve 7' are other modules such as a rapid thermal processor(RTP) reactor 49, connected to a RTP processor 45 and RTP gas box 50.Also connected to the cluster tool through a dual gate valve 7' is apair of sputter modules 46 and 47 that are used to deposit metallizationfor integrated circuit current paths. In the research environment wehave also connected an analytical module 48 including an x-rayphotoelectron spectrometer (XPS) and a static secondary ion massspectrometer (SSIMS). The limitation upon the number of module that canbe connected to the periphery of the current models of cluster tools isdetermined by the diameter of the aperture required for the dual gatevalve 7' and the size of periphery of the cluster tool. The gate valvediameter is determined by diameter of the wafers to be processed.

The preferred embodiment of window 4 is shown in FIG. 3 with greaterparticularity. The side of the window 4 facing the plasma chamber isshown to be coated by a material which is resistive to etching by thegas(es) employed in the plasma chamber. The dielectric disk 51 isfrequently quartz. However, we have discovered that this common windowsubstance is rather quickly etched by the excited H species employed inour oxide etching processes. The oxygen removed from the quartz isfrequently redeposited as an oxide on the silicon surface of the wafer10 being processed. In order to avoid this problem, it is important toeliminate as many sources of oxygen as possible from the reactor duringoxide etch. Accordingly, we have used a window which is not etchable byour plasma gases, or is coated or passivated so as to eliminateintroduction of oxygen in the chamber during etch. The ECR reactorchamber materials are stainless steel, i.e. 31655, aluminum and alumina.

In our embodiment, we employed a 3 mil thick layer 52 of a flame sprayedalumina, AlO_(x), on the microwave window quartz disk 51. This layer hasproven to permit the microwaves to pass through into the chamber withlow reflective loss and the layer is able to readily dissipate the heaton its surface and to preclude release of oxygen. In the preferredembodiment, the window cooling is also aided by the annular flowingcooling fluid at its periphery.

With the ECR equipment described in connection with FIG. 1, FIG. 2 andFIG. 3 we have discovered a plasma process employing pure hydrogen orhydrogen mixed with a small percentage of argon buffer gas, which willremove a native oxide layer rapidly, with no wafer bias, i.e., in under4 minutes with pure H₂ and under 1 minute with H₂ and 5 at % Ar, at lowtemperature, i.e., less than 200° C. The native oxide is under 25 Åthick and we have verified that essentially complete oxide removal wasaffected in these times by employing an in-situ x-ray photoelectronspectrometer (XPS) and in-situ static secondary ion mass spectrometer(SSIMS).

Through experiments, we have discovered that a 100 at % H₂ gas plasmawith a plasma pressure of 2.5 mT exhibited an unexpected peak in theproduction of excited H⁺ species, including H⁺, H₂ ⁺ and thermallyexcited neutral H₂. We have also discovered that there is a very sharpincrease in the rate of SiO₂ removal which occurs in our apparatus above600 watts of net microwave power where net power is (incident-reflected)power. We have demonstrated that our high removal rates occur with nodiscernable damage to the underlying silicon. The etch time of 4 minuteswith pure hydrogen was obtained at 1.2 KW microwave power and the etchtime of 1 minute with 5 at % Ar and 95 at % H₂ was, obtained at 1.37 KW.The operating base pressure was 7×10⁻⁸ torr. We expect that our 1 minutenative oxide etch time is a viable alternative for a commercial processsimply because of its short required exposure time.

With reference to FIG. 4, we have plotted test results taken in our ECRchamber which show a sharp peak 60 in excited H species at a particularplasma pressure, i.e., 2.5 mtorr. These results were obtained at aconstant 1.0 KW and were obtained by measuring relative emissionintensity of the hydrogen atomic 653.6 nm line. This assumes that thisline emission arises from the excitation of ground state atoms. The peak60 of FIG. 4 corresponds to the pressure at the point 61 of maximum etchrate shown in FIG. 5. The excited H species peak apparently coincideswith the maximum in the concentration of electrons, at the same plasmapressures. High electron density, i.e. greater than 1×10¹¹electrons/cm³, is apparently required to obtain the number of hydrogenions required for our process.

The effect of plasma pressure on the atomic oxygen to silicon ratioafter a 60 second ECR exposure is shown in FIG. 5. The parabolic shapedminimum 61 at 2.5 mtorr reflects the amount of oxygen removed, i.e.lowest O/Si ratio. Thereafter, the oxygen removal rate is relativelyinsensitive to pressure until around 14 mtorr when it abruptlydecreases. When the original native O/Si ratio is 0.63, it means that nooxide is being removed at those pressures. The O/Si ratio was obtainedusing a Surface Science X-ray photoelectron spectrometer. The integratedoxygen 1s transition is at 532.4 eV, normalized with respect to thesilicon concentration derived from the core-level Si2s transition at 151eV and is called the O/Si ratio after applying appropriate sensitivityfactors measured with SiO₂.

There is an unexpected discontinuity, 62, in the relationship of theO/Si ratio versus the net microwave power for a 60 sec. exposure at aconstant 2.5 mtorr pressure as shown by FIG. 6. Two distinct regimes ofoxygen removal rate are separated by a sharp transition near 600 watts.The relative insensitivity of the oxygen removal rate at the powerlevels above 1.0 KW is of practical significance in that it permits wideprocess latitude at high power.

We also noted that the surfaces etched by our process were apparentlypassivated by hydrogen bonding to the Si surface bonding sites.Specifically, an ECR H₂ plasma cleaned wafer will not absorb any O₂ orH₂ O when stored for 72 hours at 3×10⁻⁹ torr although a small amount ofcarbon absorption was detected. When exposed to room ambient for 5minutes the O/Si of our treated wafer ratio grew to only 0.025. Thisslow rate of oxide formation is beneficial in commercial handling. Anegative secondary ion mass spectrum of the ECR H₂ plasma cleaned Sisurface shows ¹ H⁻ as the most intense peak. This is an order ofmagnitude greater than the next highest peak (28 Si ¹⁶ O)⁻. Theintensity ratio of ¹ H⁻ /¹⁶ O and ¹ H⁻ /12C⁻ is 20 and 32 respectively.With regard to molecular species, the intensity ratio are ¹ H⁻ /(²⁸ Si ¹H)⁻ equal to 23 and ¹⁶ O⁻ /(²⁸ Si ¹⁶ O)⁻ equals 27. Since the verystable (¹⁶ O ¹ H)⁻ is not detected, we conclude that the large majorityof hydrogen and oxygen atoms are bonded directly to silicon and that thecleaned surface is covered with hydrogen and contaminated with less than10¹⁵ O atoms/cm² and 10¹⁶ C atoms/cm². We detect an apparent absence ofa significant amount of hydrogen diffusing into the Si lattice duringcleaning which we attribute to be a consequence of the low ion energyand the lower substrate temperature than used by prior workers.

FIG. 7 demonstrates the relationship we have discovered between the etchtime to reach an O/Si=O for H₂ plasma etch of approximately 25 Å thicknative oxide as a function of atomic percent of argon in the plasma atconstant zero bias. The point, 70, of maximum etch rate occurs at 5 at %Argon and is around 25 Å/minute. Not only have we determined that asmall amount of argon such as 5 atomic percent reduced the native oxideetch time for pure H₂ plasma from 240 seconds to 60 seconds, but we alsoobserve more stability in the plasma with this small amount of argon.Flow rates of H₂ and Ar in the experiment were 15 and 1 standard cubiccentimeters per min (SCCM) or about 7% flow rate ratio. The partialpressures in a gas mixture are a measure of their relative flow rate.The sum of partial pressures equals the total pressure. Accordingly, thesum of the hydrogen partial pressure [H] plus argon partial pressure[Ar] is equal to the total pressure. The ratio, ##EQU1## of argonpressure to the total pressure is a measure of atomic percentage.

The net microwave power was slightly higher for the Ar/H₂ gas at 1375watts for the maximum etch rate and the pressure was approximately p=4.0mtorr. We also noted for thermally produced oxide that the max. etchrate for the 5% Argon/H₂ mixture occurred at 2.2 mtorr.

Employing the improved ECR plasma chamber and processes of thisinvention has enabled us to obtain an ultra clean silicon surface whichis passivated by hydrogen. We have discovered that in conjunction with acluster tool of the type disclosed that such an ECR plasma cleanedsilicon wafer can have grown thereon an ultra pure stoichiometric Si₃ N₄film with less than 0.01 at % oxygen, hydrogen and carbon on <111> and<100> silicon. The two step process consists of cleaning the surface inan ECR excited H₂ plasma and passing the wafer, in vacuum, to a secondmodule where it is exposed to NH₃ for 2 minutes at 1070° C. to promotenitration. The films produced on <111> Si are approximately 5 nm thickwith refractive index of 2.01 at 633 nm. These films are resistant todry O₂ for six hours at 1050° C. and the average breakdown fields of acapacitor made from such films by layering Al on the surface is around 9MV/cm. In all prior work of which we are aware, Si₃ N₄ films on Si havebeen contaminated with O, H or C. The ultra purity of our layers hasbeen verified by in-situ x-ray photoelectron spectroscopy and in-situstatic secondary ion mass spectroscopy (SSIMS) used in combination withsputter depth profiling.

Our nitridation experiments were carried out in a Varian M2000 singlewafer, multi-chamber cluster tool according to FIG. 2. The silicon wafersample was a 12.5 cm diameter, n-type, 0.1 ohm-cm, <111> Si which wasexposed for 10 minutes to an ECR excited H₂ plasma. The microwave powerwas 1.0 KW and the plasma pressure was 2.5 mtorr.

After cleaning, the substrate was transferred, in vacuum, to a rapidtemperature process module 45 (FIG. 2) where it was ramped intemperature at 30° C./sec to 1070° C. and held there for 2 minutes inNH₃ at 17 torr pressure. The chamber base pressure is at 2×10⁻⁷ torr.Afterward the nitrided Si wafer was transferred in vacuum to ananalytical module where XPS and SSIMS data was taken sequentially.Transfer between modules was done at 8×10⁻⁸ torr and completed within 1minute.

A depth profile was generated by tracking N 1s, Si 2p, Si 2s, O 1s, andC 1s transitions after intermittent Ar-ion sputtering at 3 keV. Nodiscernable O 1s, or C 1s transition was observed during depthprofiling. Assuming XPS sensitivity of 0.1 at %, this result shows anorder of magnitude improvement in purity over earlier work. In addition,both positive (¹ H, ¹² C, ¹⁴ N, ²⁸ Si, ²⁹ Si, ³⁰ Si) and negative (¹ H,¹⁶ O) secondary ion mass intensities were recorded with Ar ion-gunoperating in a static mode, with current density estimated at 0.2nA/cm². The ¹² C⁺, ¹ H⁻, and ¹⁶ O⁻ intensities never exceeded 30% of thevacuum background. Assuming reasonable secondary ion sensitivity factorssuggests less than 0.01 at % impurities in the film. Circular Al/Si₃ N₄/Si capacitors, 200 μm in diameter, were fabricated in contact openingscontaining 3 μm thick 1 thermal oxide. A typical current-voltage diagrammeasured with a positive bias applied to the Al electrode wascharacterized by a breakdown field of 9 MV/cm assuming a 5 nm filmthickness. The films had current densities around 10⁻⁴ A/cm² at 3 V. Thebreakdown is as much as 3 MV/cm lower and the leakage current severalorder of magnitude higher than other thermal Si₃ N₄ layers. It is notyet clear whether this characteristic is due to the high purity of thefilm.

Another embodiment of this invention for use in connection with tungstenand titanium metallization is described as follows. Tungstenmetallization is highly desirable for high density IC fabrication.However, it has been difficult to deposit these refractory metalsdirectly on Si because of a residual layer of oxide on the active Sisurfaces. Accordingly complex silicide forming processes or depositionof WSi_(x) or T_(i) Si_(x) have been necessary as an intermediate layerbetween the silicon and the refractory metal. Because above-describedetching process results in an oxygen, carbon and nitrogen ultra oxidefree surface, we believe that we may be successful in depositing andobtaining good ohmic contacts with refractory metals without arequirement for producing or depositing an intermediate silicide.

While illustrative embodiments and applications of this invention havebeen shown and described, it would be apparent to those skilled in theart that many more modifications than have been mentioned above arepossible without departing from the inventive concepts set forth herein.The invention, therefore, is not to be limited except in the spirit ofthe appended claims.

We claim:
 1. In an ECR plasma processing apparatus having a microwavegenerator coupled to a plasma generating chamber, said plasma generatingchamber being disposed in a magnetic field, the maximum intensity ofsaid magnetic field being aligned with the axis of said plasmagenerating chamber, an interior region of said plasma generating chamberbeing isolated from said microwave generator by a dielectric microwavewindow having a first face and a second face, said first face forming aninterior wall of said interior region and said second face beingparallel to said first face, the improvement comprising:said first faceof said dielectric microwave window coated with a layer of alumina topreclude plasma etching of and release of oxygen from said first face;and said second face of said dielectric microwave window formed of aquartz dielectric material.
 2. An ECR processing apparatus according toclaim 1 wherein said layer of alumina is at least about 3 mils thick. 3.An electron cyclotron resonance reactor comprising:a microwave generatorhaving a microwave output; a reactor chamber; and said microwave outputcoupled to said reactor chamber through a dielectric microwave window,said dielectric microwave window comprising a first side forming aninterior wall of said reactor chamber and a second side parallel to saidfirst side, said microwave window formed of quartz, said first sideincluding an alumina coating having a thickness of at least about 3mils.
 4. An electron cyclotron resonance reactor according to claim 3wherein said dielectric microwave window has a constant cross sectionalshape along an axis orthogonal to said first side and said second side.5. An electron cyclotron resonance reactor comprising:a microwavegenerator having a microwave output; a reactor chamber; and saidmicrowave output coupled to said reactor chamber through a dielectricmicrowave window, said dielectric microwave window comprising a firstside forming an interior wall of said reactor chamber and a second sideparallel to said first side, said microwave window formed of quartz,said first side including an alumina coating.
 6. An electron cyclotronresonance reactor according to claim 5 wherein said dielectric microwavewindow has a constant cross sectional shape along an axis orthogonal tosaid first side and said second side.